The invention relates to diagnostic apparatus for internal combustion engines and more specifically to an improved apparatus for diagnosing and tuning engines utilizing multiple electrical igniters.
When diagnosing and tuning an engine it is desirable to know the engine speed, dwell angle - or coil charge time, and the contribution of individual cylinders to engine performance. Numerous devices that combine tachometer and dwell functions exist; these generally incorporate a meter movement with multiple scales for different ranges and engine types and sometimes a switch for the number of cylinders. The disadvantage of the above is the extra time and care required to read the correct scale and set the proper switches, for the engine under test.
Devices incorporating selective cylinder disablement to determine the contribution of individual cylinders have existed for some time. This type of device requires that a counter be synchronized with the engine in order to provide a disabling signal to the ignition circuit during the selected interval. This is accomplished by resetting the counter to its number one state by means of a signal taken from the number one ignitor wire, and advancing the counter one step for each pulse of the ignition primary signal. Due to the nature of the ignition system, (high voltage discharge) electrostatic and electromagnetic fields are produced at each spark interval that can cause the counter to reset at improper times. Although careful circuit layout and shielding can minimize this problem, there are many times in practice that the counter is "fooled" and an incorrect diagnosis results. Also the absence of a signal to the number one igniter due to an open wire or igniter will cause total loss of synchronization of the counter and again result in incorrect engine diagnosis.
All engine diagnostic equipment requires a source of power for operation. Simple meters for engine speed or dwell employing primarily passive components can acquire needed power from the ignition. As functions become more complex, a separate stable source of power is required. This is usually derived from either A.C. line current or the vehicle battery. The A.C. line supply is disadvantagous in that it requires the vehicle to be located near an A.C. outlet. The battery supply requires an extra hookup and generally limits the type of vehicle wiring system (polarity and voltage) with which the equipment will function.